This application relates to devices, techniques and material related to energy storage.
Various types of energy storage devices are available. Some example energy storage devices include chemical cells (e.g., alkaline cells, sometimes called batteries), capacitors, etc. While batteries have relatively large storage capacity, they may not be rechargeable or may take a relatively longer time to charge (e.g., several tens of minutes). Furthermore, rechargeable batteries are typically limited to less than 100 charge cycles, after which their charge holding capacity diminishes significantly. By contrast, electrostatic capacitors can be charged quickly (e.g., less than a minute) but typically have limited capacity. Carbon nanotubes (CNTs) have been proposed for electrodes in electrochemical capacitors (ECCs) or supercapacitors primarily due to the large surface area offered by CNTs and the abundance of reaction sites with the possibility of large charge storage capacity and capacitance (C). Consequently, in an electrolyte where electrochemical reactions can occur over a wide voltage range (V), large energy densities (W) per unit mass (m) can be achieved, where
  W  =                    C        ⁢                                  ⁢                  V          2                            2        ⁢        m              .  While possessing superior power densities due to the capability of fast charge/discharge, presently CNT based ECCs have lower energy densities (1-10 Wh/kg) compared to batteries (10-100 Wh/kg). With the increased used in battery operated devices such as mobile phones, tablets and the like, there is an increased need for improved energy storage devices. Furthermore, in present CNTs based ECCs, the presence of high resistance leads to low power densities.